Image scanners are commonly used in offices or homes for scanning objects such as documents, photographs or films.
Due to the some factors such as fabrication errors, uneven luminance of the lamp tube and the warm-up time period of the scanner, it is necessary to compensate the image data. As known, the uneven luminance of the lamp tube means that the luminance at both ends of the lamp tube is lower than that at the middle portion of the lamp tube. There are two conventional methods for compensating image data. The first compensating method involves in an initial calibration when the scanner is actuated. Whereas, the second compensating method relates to a no warm-up scanning operation. The operation principles of these two compensating methods will be illustrated as follows.
According to the initial calibration method, an elongated white plate is scanned to obtain a reference value. Referring to FIG. 1, a schematic top view of a white plate to be used for the initial calibration is illustrated. The white plate 100 is attached onto an edge of a scanning platform 1000 where an object to be scanned is placed thereon. In addition, the length direction of the elongated white plate 100 is perpendicular to the scanning direction A of the image scanner. That is, the elongated white plate 100 is oriented in the X-axis direction of the scanning platform 1000.
Once the luminance of the lamp tube reaches the stable state, in response to a calibrating command, the scanner begins to scan the white plate 100. Under this circumstance, the control program installed in the scanner will perform a computation on the image data of the white plate 100, thereby obtaining an X-axis calibration gain and an offset value required for compensating the image data.
The X-axis calibration gain is stored in a shading table of the image scanner. For compensation, the image data of the scanned object is multiplied by the X-axis calibration gain and the offset value is also incorporated.
The second compensating method is intended to compensate the error resulted from a reduced warm-up time of the scanner.
Referring to FIG. 2, a plot of the luminance variation of a lamp tube is illustrated. As shown in FIG. 2, the luminance of the lamp tube approaches stable after the image scanner has been turned on for a certain time period t. This time period t is referred to the warm-up time period. If a scanning operation is performed before the luminance of the lamp tube reaches the stable state, undesirable errors of the scanned image data are generated. Accordingly, for most scanners, a sufficient warm-up time period is necessary in order to achieve accurate image data.
The warm-up time period for a typical image scanner is about 30 to 90 seconds, which is time-consuming. Recently, scanning, off-line copying and printing functions are integrated into the multi-function peripheral (MFP). In addition to the warm-up time period required for performing the scanning operation, the user needs to wait for such a time period after the copy button is depressed down. This waiting time reduces the copying efficient.
Therefore, it is important to develop a method for performing the scanning operation without waiting for the warm-up time period. Some scanning methods with no warm-up time period were disclosed. For example, a compensation method comprises the steps of performing the scanning operation when the luminance of a lamp tube is unstable and then compensating the scanned image data. Such a compensation method was disclosed in a co-pending U.S. patent application Ser. No. 11/124,974, entitled “Compensation method of digital image data”, which was filed by the same assignee of the present application on May 9, 2005, and the contents of which are hereby incorporated by reference.
According to the U.S. patent application Ser. No. 11/124,974, since two compensating computations on the image data for the same scan line are necessary to perform the X-axis and Y-axis compensations, the time period for processing the image data is not sufficiently small.
In views of the above-described disadvantages resulted from the conventional method, the applicant keeps on carving unflaggingly to develop a compensation method with reduced processing time period according to the present invention through wholehearted experience and research.